Phyllodocida: History
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Daniel Martin

Phyllodocida is a clade of errantiate annelids characterized by having ventral sensory palps, anterior enlarged cirri, axial muscular proboscis, compound chaetae (if present) with a single ligament, and of lacking dorsolateral folds. Members of most families date back to the Carboniferous, although the earliest fossil was dated from the Devonian. Phyllodocida holds 27 well-established and morphologically homogenous clades ranked as families, gathering more than 4600 currently accepted nominal species. Among them, Syllidae and Polynoidae are the most specious polychaete groups. Species of Phyllodocida are mainly found in the marine benthos, although a few inhabit freshwater, terrestrial and planktonic environments, and occur from intertidal to deep waters in all oceans.

  • polychaetes
  • species diversity
  • molecular diversity
  • phylogeny
  • ecology
  • distribution

1. Introduction

The order Phyllodocida was first formulated as monophyletic by Dales [1], based on the muscular pharynx shared by the members of the clade. Later it was supported based on characters such as the ventral position of sensory palps, the presence of anterior enlarged cirri, the loss of dorsolateral folds (i.e., protrusible dorsolateral walls of the foregut), the presence of an axial muscular proboscis and the presence of compound chaetae with a single ligament [2]. It was further considered paraphyletic [3], but its monophyly has been recovered with strong support by modern molecular approaches [4][5]. Phyllodocida, like most soft-bodied polychaetes, is not well represented in the fossil record, but fossil specimens have been described from several families [3][6]. Most extant groups first appear in the Carboniferous [7], although Arkonips topororum Farrell & Briggs, 2007 [8], from the Devonian, seems to group within Phyllodocida, thus possibly constituting the earliest known member of the clade.

As currently delimited, Phyllodocida is one of the largest and most diverse polychaete clades. It includes Aphroditiformia, Glyceriformia, Nereidiformia, and Phyllodociformia, each with several generally well-defined clades interpreted with the taxonomic hierarchical level of family, but also several unplaced taxa (as Phyllodocida incertae sedis). In total, Phyllodocida holds more than 6600 species-level taxa, of which around 4627 are currently considered valid in the World Polychaeta Database (WPD) [6], where 28 families (excluding Pholoidae, a synonym of Sigalionidae [9]) and 566 valid genera are listed. However, higher taxa are constantly being revised as more insights from molecular methods and new morphological data (e.g., through microCT, confocal imaging, etc.) become available [10][11].

Phyllodocida is a ubiquitous group of annelids. Most members are typically marine benthic, but a small fraction also inhabit brackish waters, freshwater, and even terrestrial environments, and a few are holoplanktonic [12]. Benthic forms live as in- or epifauna in muddy and sandy bottoms, mixed sediments, under rocks, or hiding in crevices in hard surfaces, from shallow littoral to the deepest marine bottoms [3], including extreme environments such as hydrothermal vents [13][14][15]. Most species are free living (especially within Nereidiformia and Phyllodociformia), some burrowing in sediments (e.g., Glyceriformia), and some are tubicolous (e.g., some Aphroditiformia) [12]. Most species are ‘active-searching’ or ‘sit-and-wait’ predators, feeding on other invertebrates (e.g., among Nereidiformia, Phyllodociformia, Glyceriformia, or Aphroditiformia); some may be carrion-feeders and herbivorous, rarely alternate these trophic guilds with filter feeding (e.g., among Nereidiformia) [12]. Moreover, a large number of species (particularly within Aphroditiformia) live symbiotically with other benthic species (including echinoderms, cnidarians, decapods and, even, other polychaetes) [16][17].

Phyllodocida are among the most phylogenetically diverse groups of organisms [18][19], while the key roles they play in marine ecosystems lead them to be a demanding component for morphology-based biomonitoring [20], but also in regular and large-scale biomonitoring initiatives based on molecular tools like high-throughput sequencing [21][22]. Taking this into account, we have analysed the ocurrence records for the species of Phyllodocida included in Ocean Biodiversity Information System (OBIS) [23] to asses their biogeographic distribution and the relevance of non-indignous species, and we have analyzed all public Barcode Of Life Data System (BOLD) [24] data to assess the worldwide DNA barcode coverage for the species of the Order. This has allowed us to evaluate taxonomic uncertainties, as well as to analyze species phylogenetic diversity, to improve DNA metabarcoding studies at the taxonomic assignment step [25] and to highlight the existing knowledge gaps and the main still-pending taxonomic revisions.

2. The Diversity of Phyllodocida (Annelida: Errantia), with a Focus on Glyceridae, Goniadidae, Nephtyidae, Polynoidae, Sphaerodoridae, Syllidae, and the Holoplanktonic Families

  • The highest numbers of species of Phyllodocida have been reported from European, North American, and Australian waters, although these numbers are biased by an increased sampling effort in these regions and do not reflect true species richness. DNA barcode data show similar patterns, but also similar bias.
  • At the family level, the highest number of distribution records are for Nephtyidae, Phyllodocidae, Syllidae, Nereididae, and Polynoidae and widest distribution ranges were for Phyllodocidae, Polynoidae, Nereididae, Syllidae, and Lopadorrhynchidae.
  • Overall, there is a weak latitudinal gradient in species richness, with a rather uniformly high diversity across tropical and temperate latitudes and a drop only in extreme latitudes.
  • Antarctic and Pacific coasts of America and Asia, together with the circumtropical areas worldwide show the highest level of endemism, while the lowest numbers occur in temperate Atlantic areas and in the Arctic Ocean.
  • Most records of Phyllodocida and the highest species number and barcode data come from the high subtidal, where Nereididae, Syllidae, and Nephtyidae dominate. However, members of Phyllodocida seems to be well adapted to deep waters, particularly polynoids.
  • Less than 3% of the known species have been reported as occurring non-natively in certain parts of the world, most of them in Mediterranean waters, and more than half belong to Nereididae and Syllidae. However, many “non-native” or “introduced” species, particularly those belonging to critic species-complexes, turned to be native species with locally restricted populations when carefully examined. None of them has been considered as invasive or as pest to date.
  • There is a still unknown number of possible cryptic species complexes, this being a recurrent trend in most examined families.
  • Most examined families except to some extent Glyceridae and Goniadidae, show no traces of stabilization of the accumulative curve of species description, indicating that more new species are expected to be described in the coming years. Sources of new species diversity are mainly related with cryptic species complexes, but also with sampling in poorly explored regions and environments, with the deep-sea being particularly promising.
  • Only 620 species of Phyllodocida have sequences published in BOLD, for 1215 BINS as a consequence of having sequences (1) assigned to higher taxonomic ranks (genus or family), and (2) with wrong taxonomy assignments, the latter representing 22% and including sequences either misidentified and/or with invalid, misspelled, or synonymized names.
  • Our analyses show the key importance of keeping barcode libraries adequately curated, together with the need of adding metadata, while highlighting the apparent difficulty of having molecular data with correct identifications among Phyllodocida, with less than 60% of the records being usable at the species-level in statistical analysis.
  • Despite the amount of knowledge on the systematics of Phyllodocida, we would like to stress that there are still many open questions regarding the correct phylogenetic placement of most taxa (at different levels) so that further efforts must be dedicated to collecting new materials, allowing precise morphological descriptions in parallel with sequences.
  • We would like to highlight that there is a similar lack of knowledge with respect to the ecology of most species of Phyllodocida, as well as on their functional role in marine ecosystems all over the world oceans.
  • Taking into account that we are entering in the 2020s Oceans Decade, during which marine ecosystems have to be re-evaluated from many different points of view (from basic science to sustainable ecosystem services and derived benefits), having a real and accurate picture of the world oceans emerges as a strategic pillar, with the knowledge on the diversity they hold being keystone.

This entry is adapted from the peer-reviewed paper 10.3390/d13030131

References

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  14. Zhang, Y.; Chen, C.; Qiu, J.-W. Sexually dimorphic scale worms (Annelida: Polynoidae) from hydrothermal vents in the Okinawa Trough: Two new species and two new sex morphs. Front. Mar. Sci. 2018, 5, 112.
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